This invention is generally directed to the art of trunked radio repeater systems. It is more particularly directed to such systems using digital control signals transmitted over a dedicated control channel while also using plural working channels which are assigned temporarily for use by individual radio units.
The trunking of radio repeaters is well-known. Early trunking systems used analog control signals while some more recent systems have utilized digital control signals. Control signals have been utilized on a dedicated control channel and/or on different ones of the working channels for various diferent reasons and effects. A non-exhaustive but somewhat representative sampling of prior art publications and patents describing typical prior art trunked radio repeater systems is identified below:
U.S. Pat. No. 3,292,178--Magnuski (1966) PA1 U.S. Pat. No. 3,458,664--Adlhoch et al (1969) PA1 U.S. Pat. No. 3,571,519--Tsimbidis (1971) PA1 U.S. Pat. No. 3,696,210--Peterson et al (1972) PA1 U.S. Pat. No. 3,906,166--Cooper et al (1975) PA1 U.S. Pat. No. 3,936,616--DiGianfilippo (1976) PA1 U.S. Pat. No. 3,970,801--Ross et al (1976) PA1 U.S. Pat. No. 4,001,693--Stackhouse etal (1977) PA1 U.S. Pat. No. 4,010,327--Kobrientz et al (1977) PA1 U.S. Pat. No. 4,012,597--Lynk, Jr. et al (1977) PA1 U.S. Pat. No. 4,022,973--Stackhouse etal (1977) PA1 U.S. Pat. No. 4,027,243--Stackhouse etal (1977) PA1 U.S. Pat. No. 4,029,901--Campbell (1977) PA1 U.S. Pat. No. 4,128,740--Graziano (1978) PA1 U.S. Pat. No. 4,131,849--Freeburg et al (1978) PA1 U.S. Pat. No. 4,184,118--Cannalte et al (1980) PA1 U.S. Pat. No. 4,231,114--Dolikian (1980) PA1 U.S. Pat. No. 4,309,772--Kloker et al (1982) PA1 U.S. Pat. No. 4,312,070--Coombes et al (1982) PA1 U.S. Pat. No. 4,312,074--Pautler et al (1982) PA1 U.S. Pat. No. 4,326,264--Cohen et al (1982) PA1 U.S. Pat. No. 4,339,823--Predina et al (1982) PA1 U.S. Pat. No. 4,347,625--Williams (1982) PA1 U.S. Pat. No. 4,360,927--Bowen et al (1982) PA1 U.S. Pat. No. 4,400,585--Kamen et al (1982) PA1 U.S. Pat. No. 4,409,687--Berti et al (1983) PA1 U.S. Pat. No. 4,430,742--Milleker et al (1984) PA1 U.S. Pat. No. 4,430,755--Nadir et al (1984) PA1 U.S. Pat. No. 4,433,256--Dolikian (1984) PA1 U.S. Pat. No. 4,450,573--Noble (1984) PA1 U.S. Pat. No. 4,485,486--Webb et al (1984) PA1 U.S. Pat. No. 4,578,815--Persionotti (1985) PA1 (a) Transmission Trunked Mode in which the working channel is de-allocated as soon as the calling unit unkeys, and PA1 (b) Message Trunked Mode in which the working channel is de-allocated "n" seconds following a unit's unkeying, unless another unit keys onto the channel within such "n" seconds. "n" is called the "hang time".
Bowen et al is one example of prior art switched channel repeater systems which avoid using a dedicated control channel--in part by providing a handshake with the repeater site controller on a seized "idle" working channel before communication with the called unit(s) is permitted to proceed.
There are many actual and potential applications for trunked radio repeater systems. However, one of the more important applications is for public service trunked (PST) systems. For example, one metropolitan area may advantageously utilize a single system of trunked radio repeaters to provide efficient radio communications between individual radio units within many different agencies. Each agency may, in turn, achieve efficient communication between individual units of different fleets of sub-units (e.g., the police department may have a need to provide efficient communications between different units of its squad car force, different portable units assigned to foot patrolmen, different units of detectives or narcotics agents and the like). Sometimes it may be important to communicate simultaneously to predefined groups of units (e.g., all units, all the squad cars, all of the foot patrolmen, etc.). At the same time, other agencies (e.g., the fire department, the transportation department, the water department, the emergency/rescue services, etc.) may be in need of similar communication services. As is well-known to those familiar with trunking theory, a relatively small number of radio repeaters can efficiently service all of these needs within a given geographic area if they are trunked (i.e., shared on an "as-needed" basis between all potential units).
This invention also is especially adapted for special mobile radio (SMR) trunked users. Here, an entrepreneur may provide a trunked radio repeater system at one or more sites within a given geographic area and then sell air time to many different independent businesses or other entities having the need to provide efficient radio communication between individual units of their particular organization. In many respects, the requirements of an SMR user are similar to those of a PST user.
In fact, the potential advantages of trunked radio repeater systems for public services is so well recognized that an organization known as the Association of Public-Safety Communications Officers, Inc. (formerly the Association of Police Communications Officers) (APCO) has developed a set of highly desirable features for such a system commonly known as the "APCO-16 Requirements." A complete listing and explanation of such requirements may be found in available publications known to those in the art.
One of the APCO-16 requirements is that any user must have voice channel access within one-half second after engaging a push-to-talk (PTT) switch. This same requirement must especially be met in emergency situations--and that implies that the system must be able to actively drop lower priority users also within a very short time frame. And, of course, the ability to quickly and efficiently drop channel assignments as soon as channel usage is terminated is also important for efficient usage of the trunked facility even in non-emergency situations.
Prior trunked radio systems have attempted to more or less "just meet" such APCO-16 requirements of timeliness. For example, published specifications of one such prior system indicates an ability to achieve channel update (in a 19 channel system) within 450 milliseconds and channel drops within 500 milliseconds. To achieve this, it utilizes 3,600 bits per second (bps) digital signalling over a dedicated digital control channel. Unfortunately, although theoretically the APCO-16 requirements of timeliness should be met by such a prior system, in reality, the APCO-16 timeliness requirements are often not met--or, are met only at the expense of suffering with the obviously adverse effects of somewhat unreliable digital control signalling (which are, at best, annoying even in non-emergency situations). Accordingly, there is considerable room for improvement.
The present invention provides substantial improvements--both in timeliness and in reliability of critical control signalling in a digitally trunked radio system of this general type. To begin with, a much higher digital signalling rate (9600 bps) is utilized. However, rather than using all of the increased signalling rate to provide a 9600/3600=2.67 improvement factor in timeliness, a large portion of the increased signalling rate capacity is utilized to improve signalling reliability. Accordingly, the increased timeliness of 19 channel updating capability, for example, is improved by a factor of approximately 1.58 (e.g. 285 milliseconds versus 450 milliseconds) while the rest of the increased signalling capacity is utilized to increase the reliability of control signalling. At the same time, virtually all of the increased signalling capacity is utilized to improve the timeliness of channel drop ability (e.g., 190 milliseconds versus 500 milliseconds).
As previously demonstrated by Bell System Technical Journal articles on the AMPS system (e.g. "Voice and Data Transmission", by Arredondo et al, The Bell System Technical Journal, Vol. 58, No. 1, January 1979, pp 97-122), digital data rates on radio channels should be either very low (e.g., 200 hertz) or as high as the channel bandwidth permits. The present invention utilizes the maximum high speed data rates (e.g., 9600 bps on the typical 25 KHz bandwidth radio channel) for critical control channel signalling and control signalling on the working channels both immediately before and immediately after the user communication interval. In addition, sub-audible low-speed digital data is also utilized on the working channel during user communications so as to assure additional signalling reliability--and to also permit implementation of additional features.
In the exemplary embodiment, all channels (the control as well as working channels) are fully duplexed so that there may be simultaneous in-bound and out-bound signalling on all channels. In general, this invention achieves reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use by individual radio units as specified by digital control signals on the control channel.
Channel assignment is initially requested by a calling radio unit passing digital request signals to a control site over the active control channel. In accordance with channel availability, a controller at the central site assigns a specific then-available working channel to the requested communication and passes digital assignment signals out-bound over the control channel. Both the calling radio unit and the called unit(s) detect the working channel assignment and switch their transmitter and receiver operations over to the proper working channel. Thereafter, digital handshake signals are again exchanged between the control site and at least one of the radio units (e.g., the calling unit) over the assigned working channel. In response to a successful handshake on the assigned working channel, the central site then transmits digital release signals over the assigned working channel so as to release the appropriate units for communication thereover.
As one technique for increasing reliability, the initial request signals may include three-fold data redundancy (at least for critical data) while the channel assignment signals subsequently transmitted over the control channel may include as much as six fold redundancy of data (e.g, at least of critical data such as that representing the called party and the assigned channel). The handshake signals subsequently exchanged on the assigned working channel may also include three-fold data redundancy of at least critical data. In this manner, some of the increased signalling capacity made available by the high-speed data rate (e.g., 9600 bps) is sacrificed in favor of more reliable channel allocation and communication functions--while still comfortably exceeding all APCO-16 requirements.
To insure responsiveness to higher priority calls, sub-audible digital channel assignment update messages are also transmitted over the assigned working channel. These are monitored in each unit then residing on the working channels. Accordingly, if a higher priority call is directed to some unit already engaged in a communication, that unit is enabled to promptly switch operations to a new assigned working channel so as to immediately receive the higher priority call.
In addition, to accommodate late entry of called parties to an ongoing communication, digital channel assignment "late entry" messages continue to be transmitted on the control channel even after a successful channel assignment process has been effected so that late entrats (e.g., those just turning on their radio, just passing out of a tunnel or from behind a building or otherwise back into radio communication after temporary interruption, completion of a higher or equal priority call, etc.) may nevertheless be switched onto the proper assigned working channel as soon as possible. (The late entry feature, per se, is related to copending commonly assigned application Ser. No. 725,682, now U.S. Pat. No,. 4,649,567, filed 22 Apr. 1985.)
To effect prompt and reliable termination of channel assignments, when the PTT switch of a calling unit is released, it sends a digital unkeyed message on the assigned working channel and, in response to reception of this unkeyed message at the control site, a digital drop signal is transmitted over the assigned working channel so as to immediately drop all units therefrom and thus free that working channel for reassignment. (As will be appreciated, a given radio unit will automatically revert to monitoring the control channel upon dropping from an assigned working channel.)
The system of this invention is sometimes termed a "digitally" trunked system because trunking control is effected by digital signals passed over a continuously dedicated "control" data channel. All units are programmed so as to automatically revert to the predetermined primary control channel upon being turned on or being reset. If the expected control channel data format is not there discovered, then alternate possible control channels are successively monitored in a predetermined sequence until an active control channel is discovered. In this manner, the usual control channel apparatus at the central control site may be temporarily removed from service (e.g., for maintenance purposes). This same feature also permits continued trunked system operation in the event that the regular control channel unexpectedly malfunctions or is otherwise taken out of service.
The exemplary embodiment of this invention is designed in such a way that it meets and, in many areas, surpasses all of the existing APCO-16 requirements. It may also support available voice encryption techniques, mobile digital data terminals (digital data may be passed in lieu of analog voice data during a trunked radio communication session) and/or available automatic vehicular location systems. Preferably, a fault tolerant architecture is used [see related copending commonly assigned application Ser. No. 057,046, filed concurrently herewith] so as to maintain trunked system operation even if the central processor happens to fail at the control site. If digital data is to be communicated between radio units and/or the central site, then it may be processed through the system in a manner similar to analog voice signals. In particular, such digital data communication will be carried out at a rate accommodated within the existing audio pass band and will be trunked just like desired voice communications (i.e., no dedicated digital data communication channels are required). To help increase reliability of digital data communications, data transmissions (and analog voice transmissions, as well) may be voted in voting systems employing satellite receivers connected to the central control site.
In the exemplary embodiment, digital control signalling messages of the following types are utilized:
______________________________________ Channel Type Direction Rate ______________________________________ Control INBOUND 9600 pbs Channel Group Call Special Call Status Status Request/Page Emergency Alert/Group Call Individual Call Cancel Dynamic Regroup Dynamic Regroup - Forced Select Dynamic Regroup - Option Deselect Dynamic Regroup - Option Select Login/Dynamic Regroup Acknowledge Logical ID Request Programming Request OUTBOUND 9600 bps Channel Assignment Channel Update Enable/Disable Unit Dynamic Regroup Preconfiguration Alias ID Unit Keyed/Unkeyed Emergency Channel Assignment Cancel Dynamic Regroup Dynamic Regroup - Forced Select Dynamic Regroup - Optional Select Dynamic Regroup - Optional Select Assign Group ID An Alias ID Assign Logical ID An Alias ID Status Acknowledge/Page Time Mark Emergency Channel Update Site ID System Operationa Made Site Status Logical ID Assignment Programming Channel Assignment Working INBOUND 9600 bps Channel Initial Handshake Special Call Signalling Unit ID-PTT and Reverse PTT Miscellaneous OUTBOUND 9600 bps Initial handshake Channel Drop Status Messages INBOUND Low Speed Confirm Unit PTT OUTBOUND Low Speed Priority Scan Falsing Prevention ______________________________________
Some of the general features of the exemplary embodiment and expected benefits are summarized below:
______________________________________ FEATURE BENEFIT ______________________________________ VERY SHORT AVERAGE Practically instan- CHANNEL ACCESS TIME taneous access doesn't cut off syl- lables Normal Signal Strength Provides operation Areas: 280 Milliseconds which is faster than Weak Signal Areas (12dB most coded squelch systems. Sinad: 500 Milliseconds LATE ENTRY Minimizes missed Should a mobile turn on conversations, keeps during the period in which police up to the its group is involved in minute, minimizes a conversation, the mobile call backs. will automatically be directed to join that conversation. AUTOMATIC CHANNEL SWITCHING Frequency coordi- Mobiles, portables, control nation of the fleets stations, and consoles and subfleets automatically switch to requires no action the appropriate channel. on the part of any field personnel. HIGH SPEED CALL PROCESSING Dedicated control Processor assigns unit channel provides initiating the call, and more rapid channel all called units, to an assignments on larger systems. appropriate working channel. System size does not Initial channel assignment impact upon channel communication between site acquisition time. controller and radio units Control channel is available for occurs on the control additional functions channel. such as status and unit ID. CALL RETRY Eliminates the need Calling unit will for repetitive PTT automatically repeat its operations by the request up to eight times operator in weak signal situations. if no response is received. Terminating retries Retries terminated upon also shortens the system response. signalling time. UNIT DISABLE In hostage situa- Trunked units can be tions, these units can disabled on an individual be assigned to a basis. These disabled units special group for continue to monitor the communication with the criminals. control channel and can be Also, with automa- polled to determine their tic vehicular loca- status. tion, these units could be tracked for apprehension. SUPPLEMENTARY CONTROL Provides user and CHANNEL FUNTIONALITY system operator with system In addition to providing manager features channel assignments, the not available on control channel is used other types of for: status messages, systems. polling, system status, logging, late entry, dynamic regrouping, system testing and other system functions. GROUP PRIVACY Each group has the Each group hears only his same privacy as own group, unless having their own channel with the specifically programmed additional benefits otherwise. Dispatcher of being regrouped can override for individual with other comple- menting functions units or groups of mobiles for emergency oper- at any time. ations. CALL QUEUING Maintains orderly When all channels are entry procedure for busy, call requests will busy system. Call be queued until a channel requests are accep- ted in the order becomes available. Unit they are received requesting a channel will except higher prior- ity users go to the be notified to prevent head of the line. call backs. Members of groups already in the queue will not be reentered in the queue. DATA COMMUNICATIONS This feature greatly System has the optional enhances the value capability of using 9600 of the system be- cause it avoids the bps data on the working expense of addi- channels. Data tional RF channels. communications will take place on any equipped working channel and they are trunked, just as voice communication. VOICE ENCRYPTION Voice encryption System has the optional offers the same encrypted range as capability of using for clear voice available 9600 baud voice transmissions. Voice encryption. (See, e.g. can be passed from commonly assigned US each site through U.S. Pat. No. conventional voice 4,622,680 grade phone lines and application Serial or microwave lines Nos. 661,597 filed 17 Only minor modi- fications are needed October 1984, 661,733 to the base station filed 17 October 1984 and 661,740 filed 17 October 1984.) interface equipment to accommodate such sophisticated voice security systems. Any mobile can be upgraded to this capability by adding an external module. No internal changes to the radio are required. UNIT IDENTIFICATION Each unit on each All units are automatically transmission is identified when they identified by the transmit. This is true same ID regardless regardless of whether the of the agency, fleet transmission takes place or subfleet in which on the control channel he is currently or on a working channel. operating. 4095 is more than twice The system is able to the logical number accommodate 4095 discreet of users in a fully addresses independent of loaded twenty fleet and subfleet. channel system so there is more than sufficient capacity. TRAFFIC LOGGING Statistics on system All system information is usage, such as peak logged. Each unit trans- loading, individual mission causes the units and group usage versus time as well ID, agency, fleet, sub- as many other fleet, channel, time, site system parameters, and list of sites involved are available for to be logged for manage- tabulation analysis. ment reports. TELEPHONE INTERCONNECT All mobiles and portables in the Authorized mobiles have system can be inter- the ability to place and connected to the receive calls and patch telephone system. them to individuals or Those mobiles not specifically equip- groups of mobiles which ped for this can be may or may not be equipped patched by their dis- for telephone interconnect. patcher to maintain adequate control of system loading factors. GROUP PRIORITY ASSIGNMENT System manager can Eight priority levels are set individual group provided in the system. priorities according Each group (as well as to the criticality of each individual) is their service. assigned a priority. The flow of traffic is more easily main- tained by providing recent users priority over nonrecent users of the same priority level. ______________________________________
In the exemplary system, 11 bits are available to determine the address of a unit within an agency, fleet or subfleet. Twelve bits are available to determine the individual identity of a particular unit (i.e., its "logical ID"). The use of 11 bits for determining group addresses within an agency, fleet or subfleet provides great flexibility such that each agency may have many different fleet and subfleet structures. Furthermore, unit identification is not limited by a particular fleet and subfleet structure. The 4,096 unit identification codes can be divided among the subfleets in a manner that best suits a particular system.
Some features of this exemplary system which are believed to be particularly unique and advantageous are summarized briefly below (order of appearance not reflecting any order of importance--nor is this list to be considered in any way exhaustive or limiting):
(a) Widening the Retry Window
If a requested working channel assignment is not achieved, the request is automatically retried--and the time window in which such a retry is attempted is increased in duration as a function of the number of prior unsuccessful retries. This significantly decreases the average channel access time where noise is the real problem rather than request collisions--while still providing a recovery mechanism for request collision problems as well.
(b) Better Use of Subaudible Signalling
Rather than using subaudible signalling only to confirm channel assignements, a simple counter field is employed to greatly simplify such validity checking functions and to thus free the majority of the subaudible signalling capacity for other uses--e.g., a priority scan. In the exemplary embodiment a two bit subaudible "count" field for a given channel is incremented upon each new working assignment of that channel. Thus, if a radio unit observes a change in this field, it is programmed to immediately drop back to the control channel.
(c) Minimizing Priority Communique Fragmentations by Dynamically Altering Scan Functions
After initiating a priority call, a radio temporarily (e.g., for two seconds) disables the usual multiple group scan on the control channel--in favor of looking for the highly probable returned higher priority call. This reduces the possibility of getting diverted momentarily into an ongoing lower priority communique--and also perhaps missing a fragment of the next higher priority communique. A similar temporary (e.g., two seconds) scan preference (except for priority calls) for a just-previously involved call group also helps prevent fragmentation of non-priority communiques.
(d) Use of Transmission-Trunked Bit in Channel Assignment
The trunking system has two trunking modes:
By dynamically insuring that both called and calling units "know" that a transmission-trunking mode is in effect, the calling unit may immediately revert to the control channel upon PTT release--thus immediately freeing the working channel for drop channel signalling from the control site. The called units can also be positively prevented from ever transmitting on the working channel--thus avoiding multiple keying of radio units on the working channel.
(e) Automatic Addressing of Immediately Returned Calls
Both the called and calling units/groups are identified in the initial channel assignment signalling. The called unit captures the calling unit ID and is enabled to automatically address a return call to the just calling radio if the PTT switch is depressed within a predetermined period (e.g., 5 seconds) after the just completed communique even if the system is in the Transmission Trunked Mode. Not only does this simplify the necessary call back procedures and minimize access times, by allowing greater application of the Transmission Trunked Mode it also increases the probability of successful message exchanges--especially in poor signalling areas.
(f) 9600 bps Permits "Loose" Synchronization
Use of higher rate 9600 bps signalling permits simplified bit synchronization to be rapidly achieved by simple "dotting" sequences (i.e., a string of alternating ones and zeros 101010 . . . ). Thus, there is no need to keep information transfers precisely synchronized across all channels. This not only reduces hardware requirements system-wide, it also facilitates a more fault tolerant architecture at the control site.
(g) Improved Channel Drop Signalling
The drop channel signalling is simply an extended dotting sequence. Therefore, each radio may easily simultaneously look for drop channel signalling and channel assignment confirmations. This means that the control site may more immediately consider a given working channel available for reassignment--and, if "loaded up," immediately interrupt the drop-channel signalling to issue fresh channel assignment confirmation signals on the working channel (which each individual radio will ignore unless properly addressed to it). As a result, a "loaded" system (i.e., one where existing channel requests are already queued) may drop a working channel within about only 100 msec--and immediately reassign it to a queued request. Radios that happen to enter late into the call being dropped can detect that fact and properly drop from the channel because of the ability to simultaneously look for drop channel signalling and channel assignment confirmation signalling.
(h) Feature Programming
To avoid cumbersome feature programming (and reprogramming to add features) by factory or distribution personnel, novel procedures are employed which safely permit the end user to perform all such "programming." All units are programmed at the factory to perform all available functions. A function enable bit map and a unique physical ID are together encrypted at the factory and provided to the use as "Program Codes." When the user programs each device, its encrypted "Program Codes" are input to a Radio Programmer which, in turn, properly sets the feature enable bit map in a connected radio unit--and the decoded physical ID--and a "Just Programmed" bit). The "just programmed" radio device logs into the central controller with a request for a logical ID--based on its apparent physical ID. If illegal copying of function enabling Program Codes occurs, then the same logical ID will be assigned--and the usefulness of the radio within the trunked repeater system will be diminished.
(i) Double Channel Assignment Handshake--One Being on the Assigned Working Channel
A first 9600 bps channel assignment signalling exchange occurs on the control channel. However, a confirmation (i.e., a second handshake) then occurs on the assigned working channel. Thus, it is assured that the desired channel has been successfully assigned and locked onto before the central controller unmutes the called units on the assigned channel. The signalling is such that if the channel conditions are unsuitable for voice, the handshake will fail, thus terminating the call automatically.